A&P Recommendations for a 3D modeling app?

I know this doesn't fall into the strict definition of "art", but my younger brother is getting involved in skinning and mesh design for a couple of games that he plays, and he's also showing some interest in architecture, so I want to hook him up with a program that he can use to -- for lack of a better definition -- "do 3D modeling stuff". I think he's using a trial version of MilkShape right now, but that will expire in a few weeks.